Apparatus for volatilizing zinc



Marc'h 12, 1935. w. MCG. PEIRCE Er AL 41,994,356

APPARATUS FOR VOLATILIZING ZINC 1 Filed May 12, 195s 2 Sheets-sheet 1`origina INVENTORS ATTORNEYS March 12, 1935.

w. MCG. PElRcE ET /xLA APPARATUS FOR VOLATILIZING ZINCl Original FiledMay 12, 1933 2 Sheets-Sheet 2 INVENTORS ATTRNEYs Patented Mar. 12, 1935PATENT ol-FlcE APPARATUS Foa vom'rnrzmc zrNc Willis McGerald Peirce andRobert Kerr Waring, Palmerton,l Pa., assignors to The .New JerseyZlncCompany, `New York, N. Y., a corporation of New Jersey Originalapplication May 12, 1933, Serial No.

Divided and this application July i9, 1934, Serial N0. 735,940

1o claims. j (C1. 26e-19)' This invention relates to the volatilizingof* zinc 'metal and has for its object the provisionof i an` apparatusfor Volatilizing zinc metal. Thev in- 'fvention particularly aims toprovide improvements 5 in 'apparatus for use in volatilizing relativelylarge vquantities of zinc metal in a substantially continuousoperation.' While the invention is applicable wherever zinc metal is tobe volatilized, as for I exam le in the manufacture of French processzinc'A o ide, it can be applied with particular advantage inredistilling impure zinc metal in the purification thereof by controlleddistillation or by rectification treatment.

The present application is a division of our application Serial No.670,648, filed May12, 1933. We have found that the rate at which zinc 1can be vaporized in a refractory container depends on the percentageofthe area of the heated wall which has liquid zinc in contact with itsinner surface. This contact maintains the inside wall temperature atapproximately the boiling point of zinc and thereby increases thetemperature gradient between the heating chamber temperature and theinside wall temperature. 'Ihe increased temperature gradient increasesthe rate of heatA transfer from the heating medium to the zinc to bevaporized.

In accordance with the present invention, vthe liquid zinc is dispersedin a plurality of baths so that a high percentage ofthe inner wall ofthe refractory container in which the zinc is volatilized is in contactwith liquid zinc, and so that no large individual area of inner wallsurface is -free from contact with liquid zinc. Thus, in one of itsrpractical aspects, the invention is characterized by heating a seriesof vertically spaced pools or baths of moltenzinc metal to a suicientlyhigh temperature to volatilize the metal, and maintaining these pools ofmolten metal by charging molten metal into the top'of the series andpermitting overflow of molten metal from pool to pool. The verticallyspaced pools of molten metal may be conveniently maintained in anupright refractory chamber. internally packed to permit counter-currentflow of descending molten metal andascending vapor. The chamber mayadvantageously be packed with superposed and vertically spacedtransverse pots or trays having appropriate openings permitting thepassage of descending molten metal and ascending vapor from tray totray.

The improved apparatus of the present invention permits of theadvantageous use of relatively large zinc volatilizing retorts or zinclboilers com; pared with the usual practice of the prior art.

Zinc metal has heretofore been commonly volatilized in horizontal orslightly inclined cylindrical retorts, generally resembling the retortsof the Belgian spelter furnace, or in somewhat larger bottle-shapedretorts. In a small retort, such as the retorts or bottles commonly usedin the prior art, the percentage of wall surface in contact with liquidzinc is limited by the necessity of maintaining a vapor space at the topof the retort. If the size of the retort is increased in order toprovide a unit of large capacity, several difliculties are encountered:

1. Retorts or bottles of -the form used in the prior art will, if madeof large size and filled to the customary degree, contain deep baths ofliquid zinc. The hydrostatic head of such deep baths of the heavy liquidzinc rapidly becomes too great for a refractory container safely tosustain.

2. A monolithic container becomes structurally impossible; and if aretort similar in form to those of the prior art is built up of separatebricks, the joints between the bricks will be, according to experience,unable to withstand the pressure of liquid zinc without leaking. f

3. Carrying out vaporization in a deep bath of a heavy liquid, such aszinc, results in increased l boiling point at the bottom and alsoresults in violent ebullition, which is objectionable because itcontamnates the vapor with a spray of impure metal.

The apparatus of the present invention obviates these diiculties andthus permits of the volatilization of zinc in units of large capacityand high output, as follows:

1. The distribution of the liquid (molten) zinc in a plurality of bathsdivides the hydrostatic head of the molten zinc so that it is notsufliciently great at any point to endanger the refractory container.

2. The apparatus ofthe present invention may be built up of individualunits in such a way that no joint is exposed to the-pressure of liquidzinc, thus obviating leakage of liquid zinc through the joints.

3. The distribution of the liquid zinc in a plurality'of baths renderseach bath relatively shallow, and thus makes negligible the increase inthe boiling point of zinc at the bottom of the bath brought about 4bythe hydrostatic head vof the liquid zinc. The relative shallowness ofthe individual baths diminishes the violence of ebullition, andfurthermore, the degreeof 'ebullition present here is notdisadvantageous in the apparatus of the present invention, in view ofthe fact that the molten metal flowing down from bath to bath 2. K :E incounter-current witlrlascending.zir1c vapor rinses out of the' zincvapor lthespraylof impure momen metal with which ebuuiticn mtaminatesthe zinc vapor.

To attain liquid metal contact with the maximum percentage of heatedwall area without unduly increasing the weight of liquid zinc to besupported by each unit of the retort or boiler, we may employ a unit inwhich a trough along each Wall holds liquid zinc in contact `with alarge proportion of the total wall area. In such a'vaporizing process asis contemplated by this invention, the impurities with higher boilingpoints and higher densities than zinc, such as lead and iron, will tendto concentrate and remain in theindividual troughs. We have discoveredtwo means of limiting this concentration by continuously removing theseimpurities from the troughs.

1. By. providing a suicient violence of ebullition, these impurities arekept in suspension and carried from one trough to the next and nally outof the last trough, together with a limited quantity of zinc.

2. By so designing the troughs and overflows that the flow of liquidmetal from one unlt'to the next carries with it such impurities, whichbecause of their greater density tend to settle out in the bottom of thetrough.

In practicing the present invention as a part of a purification systemto remove high boiling impurities, it is known that these impurities,for examplelead, are carried into the vapor stream by mechanicalentrainment of the mist due to boiling. The amounts so carried increasethe impurity content of the vapor to an amount greater than wouldotherwise be present in the vapor by direct vaporization. By suitablearrangement of the retort units, to baffle and scrub the vapor, we mayaccomplish the removal of a substantial proportion of such entrainedmist. Byv keeping the inner walls of the retort covered with liquidmetal, we may also minimize the superheating of the vapor and consequentvaporization of such mist. If such vaporization does occur to someextent, thereby increasing the content of impurity vapor in the vaporstream, a means of reabsorbing this excess impurity vapor by contactwith the liquid metal is provided.

We may also decrease the amount of entrained mist by so adjusting theproportions of the trough as to avoid violent ebullition. This may, ifdesirable, be carried to a point where the unit becomes a shallow tray,though a loss in Vaporizing capacity results.

The invention may be embodied lin various forms and types of apparatus,and in the accompanying drawings we have illustrated certain ap-fparatus with whichv we have secured excellent `resultsin the practice ofthe invention. In the drawings,

Fig. .1 is a sectional elevation of an apparatus for `volatilizing zincmetal in accordance with the principles of the invention,

Fig. 2 is a plan of one of the transverse trays of the apparatus of Fig.1,`

Fig. 3 is a sectional elevation of an` apparatus with a modified form oftransverse tray in conjunction with a lead-eliminatingrectifying co1-umn,

Fig. 4`is a plan of the transverse tray of the retort or volatilizingchamber of the apparatus of Fig. 3, l.

Figs. 5, 6, 7 and 8 illustrate still further modified forms oftransverse trays adapted as internal packing for the volatilizingchamber or retort of the invention, and

Fig. 9 is a vertical section on the line 9 9 of Fig. 5.

Referring to Fig. 1 of the drawings, there is shown an upright furnacestructure 10 having a heating chamber or laboratory 11 provided withburner openings 12 and an outlet 13 for 'the exhausted heating gases.PyrometersV 14 extend through the walls of the furnace structure intothe heating chamber 11 for indicating and recording the temperature ofthe heating gases therein. It will, of course, be understood that anyother appropriate means of heating may be used in lieu of the burnersand hot gases of combustion indicated in the apparatus of Fig. 1.

A A vertical volatilizingv chamber or retort is built within the heatingchamber 11. This retort is built up of superposed rectangular trays orpots 15 of silicon carbide or equivalent refractory material of goodheat conductivity. The top and bottom edges of the 'trays are bevelledas at 16 so that they may be stacked one above the other to form thevertical retort. The joints between the trays are made zinc-vapor-tightby an appropriate cement, such as a cement of silicon carbide'with asmall amount of clay. The bottom of each tray has a transverse opening17 approximate one end thereof. The upper surface of the bottom of eachtray is provided with transverse ribs 18 parallel to the opening 17; theribs being of progressively increasing height from the end nearest theopening to the end furthest from the opening. The trays 15 are stackedin staggered relationship with respect to the openings 1'7 so that thebottoms thereof form a column of superposed and vertically spacedbattles through which metallic vapor ascends and molten metal descendsin tortuous or zig-zag paths. The ribs 18 serve to retain shallow poolsof molten metal on the bottoms of the trays. 'I'he cascade effect ofmolten metal pouring over the ribs and falling from tray to trayfacilitates intimate contact between ascending vapor and descendingmolten metal.

The bottom of the retort is formed by a tray 19 having a lateral openingcommunicating with a discharge pipe 20, the orifice or tap hole of whichis closed by a plug 21. An outlet 22 is mounted on the top of the retortfor withdrawing the zinc vapor evolved therein. The outlet 22 isoperatively connected with any appropriate apparatus or device (notshown) for utilizing or treating the zinc vapor evolved from theretort;J At its top, the retort is provided with a charging device 24for the introduction of molten zinc metal under conditions preventingthe escape therethrough of any vapor evolved in the retort.

In operating' the apparatus of Fig. 1 in accordance with the presentinvention, molten zinc metal is introduced into the top of the verticalretort (15) through the charging device 24. As molten metal is added tothe charging device, either continuously 4or at frequent intervals, thetray or charging well thereof overows into the top transverse tray 15 ofthe vertical retort or boiler. Molten metal flows downwardly through theopenings 17 from tray to tray, and pools of molten metal are retained onthe bottoms of the trays by the ribs 18. 'Ihe vertical retort is heatedto a suiilciently high temperature by the hot gases surrounding it inthe heating chamber 11 to actively boil the shallow pools of moltenmetal held on the trays. 'I'he heated walls of the retort are wettedwith 'a plurality of baths of `that may tend to collect molten zincmetal over a relatively large proportion of the wall area, and there area plurality of baths of molten metal of relatively large surface areaexposed to radiant heat, and in consequence of these two factors(particularly the first) very eilicient volatilization of the moltenmetal takes place. Molten zinc metal is charged into the top of theretort in suicient amount to maintain poolsof molten metal on all of thetrays of the retort, and this condition' is established and maintainedwhen a small amount of molten metal continuously overflows into thebottom tray 19 of the retort. In general, the rate of feed of moltenmetal to the retort approximates its boiling capacity. Where this rateof feed of molten metal is exceeded, the excess molten metal withdrawnfrom the bottom of the retort (if not too high in impurities) may berecharged into the top.

In Fig. 3 of the drawings the vertical retort or boiler is mountedwithin a heating chamber as in the apparatus of Fig. l, but the retortis built up of trays or pots having a V-shaped peripheral trough orgroove 26 for holding molten metal. This groove is formed between theouter wall of the pot and an inner inclined wall 27 of the bottom 28 ofthe pot. It will be understood that all these parts of the tray or potconstitute an integrally molded article. The bottom 28 has an opening 29approximate one end thereof, and the upper edge of the opening has a rib30 for retaining a shallow layer of molten metal on the top surface ofthe bottom member 28.

A lead-eliminatingreflux or rectifying column is mounted above the topof the vertical retort or boiler formed by the trays (25) to receivevapor therefrom. The lead-eliminating column comprises a vertical seriesof spaced transverse trays or baffles 31 generally resembling inconfiguration, structure and mounting the trays 15shown in Figs. 1 and 2of the drawings. An outlet 32 at the top of the column formed by thetrays 31 convey zinc vapor (purified with respect to lead) to acondenser or to other appropriate apparatus for the recovery,utilization or subsequent treatment of the vapor.

Molten zinc metal to be volatilized in the retort formed by the trays(25) is introduced into one of the lower trays of the rectifying vcolumnthrough a pipe 33 communicating with a charging well 34. A continuous Astream of molten zinc metal flows into the charging well 34 through anopening 35 in the bottom of a superposed charging well 36. The amount ofmolten metal flowing through the opening 35 is regulated by adjustmentof the position of a cooperating valve stem 37. An appropriate level ofmolten metal is continuously maintained in the upper charging well 36.

Molten metal descends from tray to tray of the retort through theopenings 29. Any impurities inthe troughs 26 are kept in suspension byviolent ebullition prevailing therein and are in consequence swept outof the troughs and carrieddown through the retort by the descendingstream of molten zinc. The impurities present less Volatile than zinc,along with a limited quantity of zinc, escape from the retort throughthe discharge tube 20 and the tap-hole thereof.

The rectifying column formed by the trays (31) is thoroughly insulatedthroughout its length, except at the extreme top thereof. Thisuninsulated top of the rectifying column serves as a dephlegmator orcondenser to supply molten metal for refluxing backy through the column.The retort or boilerformed by the trays (25) operates continuously tovolatilize large quantities of impure zinc metal, and the resultingvapor is subjected toa rectification treatment in the rectifying columnfor removing lead therefrom as described in 'the copending'applicationof Messrs. Holstein and Ginder, Serial No.- 540,566, filed May 28, 1931.If desired the lead-free zinc vapor flowing from the top of therectifyingcolumn may be conducted to a cadmium-eliminating rectifyingcolumn for removing cadmium therefrom as described in the copendingapplication of Messrs. `Ginder, Peirce and Waring, Serial No. 620,634,filed July 2, 1932. y

In Figs. 5, 6 and 9, there is shown a still further modified form oftray o r pot for the Vertical retort or boiler of the invention. Thetray 38 is rectangular in section and has an inner peripheral trough orgroove 39 for holding molten metal to be volatiliz'ed. At one end of.the tray the groove 39 has spaced curtains 42 and spaced dams 40; themolten metal flows under the curtains 42 and over the dams 40, andescapes from the tray through an opening 41 in the bottom of the grooveand between the dams 40. In flowing under the curtains 42, the moltenmetal sweeps out of the grooves any impurities denser than molten zinc(such as leader iron) that may tend to collect therein. With this formof tray the volatilized zinc metal passes freely up through the opencenter of the retort while the descending molten metal flows through theopenings 41 from tray to tray and in zig-zag fashion through the grooves39 thereof.

In Figs. 7 and 8. there is showna type of tray resembling that shown inFigs. 5, 6 and 9 except that the tray is circular instead of rectangulary and is not provided with curtains. Thus, the circular tray 38' has aninner circular peripheral groove 39 with spaced dams 40 and opening 41'in the bottom of the groove and between the dams.

In the actual operationof a five foot high retort built up of ten traysof the type shown in Figs. 3 and 4, 12" by 24" external dimensions and awall thickness of 11/4f, we have volatilized zinc metal at the rate ofabout 20,000 pounds per day of 24 hours. Such a retort has an outsideheated surface of 30 square feet, so that the boiling rate per squarefoot of outside heated surface is about 666 pounds per day. This boilingrate per square foot of outsideheated surface is from 3 to 6 times asgreat as in retorts of the type heretofore commonly. used involatilizing quantities of zinc metal are tobe volatilized, and

is also adapted for the production of zinc vapor low in lead bycontrolled redistillation of zinc metal with relatively high leadcontent. For this application of the invention the internal packing ofthe retort or boiler should'give intimate contact between `the ascendingvapor and descending molten metal, such as provided in the apparatus ofFigs. 1 to 4 of the drawings .so as to remove superheat and metallicmist from the zinc vapors. The retort is charged with impure zinc metalcontaining lead and/or other contaminants with boiling points exceedingthat of zincj at a rate somewhat in excess of the boiling capacity ofthe retort. The retort may be so operated that the zinc vapor evolvedfrom the retort contains an amount of lead that approaches thatcorresponding to the partial pressure of lead in the alloy charged intothe retort at the boiling point of the alloy. The relative purity of thezinc vapor is due to the following factors:

1. The scrubbing action of the shower of molten zinc metal descendingthrough the ascending vapor tends to remove superheat and thus a por'-tion of the lead 'vapor evaporated by this superheat is condensed orredissolved, and at the same time the scrubbing action removes metallicmistfrom the ascending vapor.

2. Superheat is substantially decreased not only by the scrubbingaction, but also in consequence ofthe ation from'the walls of the retortand by the great extent to which the walls of the retort are coveredwith or scrubbed by molten metal.

Furthermore, the metallic vapor leaving the retort or boilerat the tophas on account of its intimatev contact with the descending molten metalample opportunity t'o approach closely to a state of equilibrium withthe descending molten metal. In view of this circumstance the vaporleaving the retort may have a lead content approaching closely to thelead content of vapor `in equilibrium with the molten metal charged intothe retort. The purest zinc vapor evolved in the retort is the vaporthat is evolved by ,the molten lead-zinc alloy just entering the retort.The vapor evolved from a boiling zinc-lead alloy contains a lowerpercentageof lead and a greater percentage of zinc than the moltenzinc-lead alloy. In consequence the molten zinc-lead alloy becomesgradually enriched in lead as it descends through the retort'. Moreover,the vapor evolved from the molten metal also becomes richer in lead asthe lead content of the molten metal is'enriched'during its descentthrough the retort. The metallic vapor evolved at the bottom of theretort is thus richer in lead than that evolved at the top. However, theexcess lead content in the vapor evolved from the molten metal near thebottom of the retort tends to be removed from this vapor by thescrubbing action of the descending molten metal so thatfthe lead contentof the vapor as it leaves the retort is nearly in equilibrium with theentering molten metal.

When operated in the foregoing manner, the vertical retort of theinvention is capable of producing large amounts of relatively pure zincvapor from impure zinc by distillation without refluxing or rectication,scrubbing by. the descending molten metal charged into the retort beingrelied upon to secure relatively pure zinc vapor. In many cases thiszinc vapor will be suiiicently pure for direct utilization thereof andmay be conveyed to any appliance for utilizing it, such as axcanisterfor producing zinc dust, a combustion chamber for the production of zincoxide, etc. Where a furtherI removal of lead from the vapor evolved-fromthe retort is necessary, the apparatus of Fig. 3 of the drawings withits super posed lead rectifying column is employed.

Relatively greater ebullition takes place in the troughs 26 of the potsof Fig. 3 than does on the more shallow pools held on the trays 15 ofFig. 1, The trough type of pot 25 has a greater boiling capacity thandoes the tray l5. 'I'he open-center pots of Figs. 5 to 8 of the drawingmay often be used with advantage where it is unnecessary 4or undesirableto operate the retort or boiler with particular attention to evolvingvapor of a lead content in equilibruim with the molten metal chargedinto the retort.

large bath surface exposed to heat radi-r 'said trays bzing such as Weclaim:

1. An apparatus for volatilizingv zinc metal comprising an uprightchamber'having a. plurality of integrally formed transverse superposedtrays provided with ribs on their uppersurfaces and adapted to holdpools of molten metal, each of said trays being provided with anlopening to permit the downward 110W of molten metal from tray to trayand the upward flow of vapor, and means for heating said chamber to asuciently high temperature to volatilize molten metal in said pools.

2. An apparatus for volatilizing zinc metal `comprising a built-upretort having a. multiplicity of vertically spaced integrally formedtransverse trays each of which is provided with one or more ribs on itsupper surface and adapted to hold a pool of molten metal, an openingbeing provided in each tray whereby molten metal flows downwardlythrough said retort from tray to tray and vapor iiows upwardlythroughthe retort, and means for heating the retort to a sufciently hightemperature to volatilize substantial quantities of molten metal in saidpools.

3. An apparatus for volatilizing zinc metal comprising an uprightchamber -having a plu-f rality of vertically spaced integrally formedtransverse trays, the side walls of the trays forming the side walls ofthe chamber, each ofsaid trays having a peripheral trough formedintegrally therewith and adapted to hold a pool of molten metal andmeans permitting the overflow of molten metalin said trough to the nextlower tray in the cha ber, the said overflow means being located at alevel below the top of the side walls of the tray, the arrangement of topermit an upward flow of vapor through the chamber, means near the topof the chamber for introducing molten metal, and means for withdrawingvapor evolved in the chamber.

'4. An apparatus for volatilizing zinc metal comprising a retort builtup of superposed integrally formed units each f which is adapted to holda bath of molten zinc metal, said units being so constructed between anytwo units are located above the bath of molten zinc metal in the lowerof the two units and no joint in the built-up structure is exposed tothe hydrostatic head of any bath of molten zinc metal therein.

5. An apparatus for' volatilizing zinc metal and assembled that jointsAcomprising a retort built up of superposed inteabove the pool of moltenmetal in the lower of the y two units and no joint in the built-upstructure is exposed to the hydrostatic head of any pool of molten metaltherein, means permitting the downward flow of molten metal from troughto trough and the upward flow of vapor, means near the top of the retortfor introducing molten metal, means for withdrawing vapor evolved in theretort, and means for heating the retort to a sufficiently hightemperature to volatilize molten metal in said pools.

6. An apparatus for volatilizing zinc metal which comprises a retortbuilt up of a plurality of superposed integrally for-med trays the sidesof which form the wall of the retort, damming means on the uppersurfaces of-the trays adapted to hold pools of molten zinc, and meanswhereby heat is applied to the Wall of the retort to volatilize zinctherein.

7. An apparatus for volatilizing zinc metal which comprises a retortbuilt up of a plurality of superposed integrally formed trays the sidesof which form the wall of the retort and the bottoms of which form aseries of baies, whereby ascending zinc vapor and descending molten zincare forced to pursue a tortuous path, and damming means on the uppersurfaces of the trays adapted to hold pools of molten zinc.

8. An apparatus for volatilizing zinc metal which comprises a retortbuilt up of a series of superposed integrally formed units the sides ofwhich form walls of the retort, said units having troughs adjacent thesides and formed integrally therewith adapted to retain pools of moltenzinc.

9. An apparatus for volatilizing zinc metal comprising a retort built upof superposed intef' grally formed units each of which provides acontainer formed integrally with its respective unit and so constructedand arranged as to retain a bath of molten zinc metal contacting with alarge proportion of the inner surface of the heated Wall of the retort.

10. An apparatus for volatilizing zinc metal which comprises a retortbuilt up of a plurality of superposed integrally formed units, the sidesof which form walls of the retort, said units having troughs adjacenttheir sides adapted to contain molten zinc, spaced curtains locatedwithin said troughs under which surplus molten zinc must flow in orderto leave the troughs, and overow means disposed between said curtainswhere# by the4 troughs are kept substantially full of molten zinc.

WILLIS MCGERAID PEIRCE. ROBERT KERR WARING.

